On the recent warming of the southeastern Bering Sea shelf

P. J. Stabeno1, N.A. Bond,1,2 and S.A. Salo1

2Joint Institute for the Study of the Atmosphere and Ocean (JISAO), University of Washington, Seattle, Washington 98195

Deep-Sea Research II, 54, 2599–2618
Published by Elsevier Ltd. Further electronic distribution
is not allowed.

1. Introduction

The 500-km-wide continental shelf of the eastern Bering Sea supports some
of the United States’ most productive and valuable fisheries, and immense
populations of marine birds and mammals that contribute to the subsistence
of communities of Native American peoples. It is a major economic, social,
and environmental resource for the United States. Like all high-latitude ecosystems,
the Bering Sea is sensitive to shifts in climate on temporal scales ranging
from interannual (e.g., El Niño/Southern Oscillation (ENSO))
through decadal (e.g., Pacific Decadal Oscillation (PDO) and the Arctic Oscillation
(AO)) to long-term secular trends. In fact, although the Bering Sea is dominated
by year-to-year variability, dramatic shifts in the physical, and biological
environment of the southeastern Bering Sea have occurred recently (e.g., Stabeno
and Overland, 2001; Overland
and Stabeno, 2004; Overland
and Wang, 2005a, b).
It is an open question whether these changes are due more to regional effects
or to fluctuations in hemispheric modes of variability.

Sea-ice cover, a defining
characteristic of any arctic or subarctic system, is decreasing in duration
and concentration over the southeastern Bering Sea shelf (Overland
and Stabeno, 2004), and is characterized by a faster melt back in spring over the northern
shelf (Grebmeier
et al., 2006). There are multiple possible causes for this
decrease related to changes in atmospheric forcing and oceanic conditions.
Changes in the timing and spatial extent of sea-ice impact the temperature
of the Bering Sea shelf and, in particular, the extent of the cold pool (the
region where bottom temperatures are <2°C during the summer) over the middle
shelf. They also affect the timing of the spring-phytoplankton bloom (Stabeno
and Hunt, 2002; Hunt
et al., 2002).

Such changes in the physical environment
are capable of reorganizing the ecosystem (Hunt
et al., 2002; Hunt
and Stabeno, 2002), and there is evidence that this ecosystem is changing. For instance,
certain cold-water species such as Greenland turbot (Reinhardtius hippoglossoides)
and certain amphipods are no longer found in great numbers in the southern
Bering Sea (Boldt,
2004). The biomass of jellyfish (medusae) rose markedly
in the 1990s (Brodeur
et al., 1999) and then declined rapidly beginning about
2001, with probable linkages to regional climate variations. The central feeding
location of the gray whale (Eschrichtius robustus) has shifted from the northern
Bering Sea to the Chukchi Sea (Moore
et al., 2003). At the same time, there
has been a decline in the productivity and overall benthic standing stock over
the northern Bering Sea (Grebmeier
et al., 2006). These changes have occurred
at the same time as the warming of the shelf and the decrease in sea-ice cover.

Oceanographic observations in the Bering Sea have been limited by its remoteness,
its size and the harsh weather that dominates this area, especially in the
winter. To increase the year-round observations over the southeastern Bering Sea
shelf, we established three biophysical mooring sites in 1995. Two of the sites
were only occupied for a few years, but Site 2 (M2) has been occupied nearly
continuously since 1995. The location of M2 was selected as an area where sea
ice occurred virtually every winter for at least a short period (Stabeno
et al., 1998). In 1996, a biophysical mooring was deployed farther to the northwest
in a region of the middle shelf that appeared to have a weak cross-shelf flow.
In 1999, we selected this location (M4) as a second monitoring site. Data from
both these sites have provided critical quantification of the changing physical
environment over the southeastern Bering Sea shelf.

In this article, we first
present information about the changing extent and duration of sea ice over
the southeastern shelf. Next, we relate these changes to data on temperature,
currents and fluorescence collected at several mooring sites on the southeastern
Bering Sea shelf. Finally, we discuss four possible mechanisms that likely
contributed to the observed warming over the Bering during the last decade.